In an emetropic human eye, the far point, i.e., infinity, is focused on the retina. Ametropia results when the far point is projected either in front of the retina, i.e., myopia, or in the back of this structure, i.e., hypermetropic or hyperopic state.
In a myopic eye, either the axial length of the eye is longer than in a normal eye, or the refractive power of the cornea and the lens is stronger than in emetropic eyes. In contrast, in hypermetropic eyes the axial length may be shorter than normal or the refractive power of the cornea and lens is less than in a normal eye. Myopia begins generally at the age of 5-10 and progresses up to the age of 20-25. High myopia greater than 6 diopter is seen in 1-2% of the general population. The incidence of low myopia of 1-3 diopter can be up to 10% of the population.
The incidence of hypermetropic eye is not known. Generally, all eyes are hypermetropic at birth and then gradually the refractive power of the eye increases to normal levels by the age of 15. However, a hypermetropic condition is produced when the crystalline natural lens is removed because of a cataract.
Correction of myopia is achieved by placing a minus or concave lens in front of the eye, in the form of glasses or contact lenses to decrease the refractive power of the eye. The hypermetropic eye can be corrected with a plus or convex set of glasses or contact lenses. When hypermetropia is produced because of cataract extraction, i.e., removal of the natural crystalline lens, one can place a plastic lens implant in the eye, known as an intraocular lens implantation, to replace the removed natural crystalline lens.
Surgical attempts to correct myopic ametropia dates back to 1953 when Sato tried to flatten the corneal curvature by performing radial cuts in the periphery of a corneal stroma (Sato, Am. J. Ophthalmol. 36:823, 1953). Later, Fyoderov (Ann. Ophthalmol. 11:1185, 1979) modified the procedure to prevent postoperative complications due to such radial keratotomy. This procedure is now accepted for correction of low myopia for up to 4 diopter (See Schachar [eds] Radial Keratotomy LAL, Pub. Denison, Texas, 1980 and Sanders D [ed] Radial Keratotomy, Thorofare, NJ, Slack publication, 1984).
Another method of correcting myopic ametropia is by lathe cutting of a frozen lamellar corneal graft, known as myopic keratomileusis. This technique may be employed when myopia is greater than 6 diopter and not greater than 18 diopter. The technique involves cutting a partial thickness of the cornea, about 0.26-0.32 mm, with a microkeratome (Barraquer, Ophthalmology Rochester 88:701, 1981). This cut portion of the cornea is then placed in a cryolathe and its surface modified. This is achieved by cutting into the corneal parenchyma using a computerized system. Prior to the cutting, the corneal specimen is frozen to -18.degree. F. The difficulty in this procedure exists in regard to the exact centering of the head and tool bit to accomplish the lathing cut. It must be repeatedly checked and the temperature of the head and tool bit must be exactly the same during lathing. For this purpose, carbon dioxide gas plus fluid is used. However, the adiabatic release of gas over the carbon dioxide liquid may liberate solid carbon dioxide particles, causing blockage of the nozzle and inadequate cooling.
The curvature of the corneal lamella and its increment due to freezing must also be calculated using a computer and a calculator. If the corneal lamella is too thin, this results in a small optical zone and a subsequent dissatisfactory correction. If the tissue is thicker than the tool bit, it will not meet at the calculated surface resulting in an overcorrection.
In addition, a meticulous thawing technique has to be adhered to. The complications of thawing will influence postoperative corneal lenses. These include dense or opaque interfaces between the corneal lamella and the host. The stroma of the resected cornea may also become opaque (Binder Arch Ophthalmol 100:101, 1982 and Jacobiec, Ophthalmology [Rochester] 88:1251, 1981; and Krumeich JH, Arch, AOO, 1981). There are also wide variations in postoperative uncorrected visual acuity. Because of these difficulties, not may cases of myopic keratomileusis are performed in the United States.
Surgical correction of hypermetropic keratomyclosis involves the lamellar cornea as described for myopic keratomyelosis. The surface of the cornea is lathe cut after freezing to achieve higher refractive power. This procedure is also infrequently performed in the United States because of the technical difficulties and has the greatest potential for lathing errors. Many ophthalmologists prefer instead an alternative technique to this procedure, that is keratophakia, i.e., implantation of a lens inside the cornea, if an intraocular lens cannot be implanted in these eyes.
Keratophakia requires implantation of an artificial lens, either organic or synthetic, inside the cornea. The synthetic lenses are not tolerated well in this position because they interfere with the nutrition of the overlying cornea. The organic lenticules, though better tolerated, require frozen lathe cutting of the corneal lenticule.
Problems with microkeratomies used for cutting lamellar cornea are irregular keritectomy or perforation of the eye. The recovery of vision is also rather prolonged. Thus, significant time is needed for the implanted corneal lenticule to clear up and the best corrective visions are thereby decreased because of the presence of two interfaces.
Application of ultraviolet and shorter wavelength lasers also have been used to modify the cornea. These lasers are commonly known as excimer lasers which are powerful sources of pulsed ultraviolet radiation. The active medium of these lasers are composed of the rare gases such as argon, krypton and xenon, as well as the halogen gases such as fluorine and chlorine. Under electrical discharge, these gases react to build excimer. The stimulated emission of the excimer produces photons in the ultraviolet region.
Previous work with this type of laser has demonstrated that far ultraviolet light of argon-fluoride laser light with the wavelength of 193 nm. can decompose organic molecules by breaking up their boundings. Because of this photoablative effect, the tissue and organic and plastic material can be cut without production of heat, which would coagulate the tissue. The early work in ophthalmology with the use of this type of laser is reported for performing radial cuts in the cornea in vitro (Trokel, Am. J. Ophthalmol 1983 and Cotliar, Ophthalmology 1985). Presently, all attempts to correct corneal curvature via lasers are being made to create radial cuts in the cornea for performance of radial keratotomy and correction of low myopia.
Because of the problems related to the prior art methods, there is a continuing need for improved methods to correct eyesight.